In recent years, spray products are applied to various fields, and many of these products are spray preparations obtained by mixing a mother liquid in a liquid state with a formulation that has a desired use. Prescribed properties desired for these spray products generally include (1) that excellent spraying can be achieved under any environment, (2) that a drop of the mother liquid excellently fixes to a sprayed surface without causing ununiform spraying, (3) that dripping of the mother liquid is less likely to occur on a vertical surface and an inclined surface, and (4) that a drop of the mother liquid causes no damage to a sprayed surface (skin and the like, in particular) for a prolonged period of time after the drop of the mother liquid is dried and fixed, which means the spray product is safe, and the like.
To fulfill these prescribed properties (1) to (3), various techniques have been developed until now.
For example, a technique has been developed for (3) preventing dripping by dissolving polymer thickener in a mother liquid to increase the viscosity of the mother liquid [Patent Document 1]. However, in the case of a solution of an ordinary macromolecular compound, spraying itself may be impossible when the viscosity of mother liquid is high enough to effectively prevent dripping. In order to achieve sucking into a nozzle and to maintain sprayability, the viscosity of mother liquid should be designed to be low enough, which in turn cannot adequately prevent dripping. In other words, to achieve both dripping prevention and smooth spraying is crucial. However, optimization of viscosity of mother liquid to that end has been extremely difficult. In addition, a solution of a macromolecular compound has a typical characteristic (spinnability) to easily form threads during spraying and, therefore, ideal mist in which liquid drops are independent from each other cannot be obtained. Thus, mist formation is significantly poor as compared to the case with no thickener, resulting in ununiform spraying. That is also a problem.
Many attempts have been made, in order to enhance the properties (2) and (3), to formulate a surfactant in a mother liquid to achieve thickening via formation of a micellar structure in an aqueous phase and to control the surface tension of a sprayed drop of the mother liquid [Patent Documents 2, 3].
However, for example, the mother liquid has fluidity, and therefore, it cannot be sprayed when inverted. Thus, the property (1) cannot be successfully achieved. In addition, when the surfactant content is increased in order to enhance dripping prevention, in the case of spray for skin, for example, skin irritation is likely to develop, which is inconvenient in terms of safety as of (4). That is another problem.
To fulfill the prescribed properties (1) and (3), enhancement in the structure of a container has been developed (Patent Document 4, for example). In these instances, thin mist that is fixed on a coated surface achieves an excellent result. However, heavy spraying, namely thick coating, causes dripping. Therefore, a problem in dripping prevention has not been substantially solved.
As techniques that achieve the prescribed properties (1) to (4) in a relatively good balance, sprays containing a gel composition have been disclosed in which the gel composition is predominantly composed of hectorite that is constituted of hydrophilic smectite (Patent Document 5, Patent Document 6). However, these techniques disclosed have problems that the spray is mainly composed of inorganic substances with poor track records of safety use and that the hectorite aggregates in a main dispersion medium of the spray composition, such as alcohols, to lead a decrease in spraying properties.
Under these circumstances, a spray that utilizes cellulose dispersion has been developed focusing on its sprayability, foam-forming ability, foam-retention, thixotropy, and dispersion stability. However, cellulose has poor compatibility with fat-soluble agents and other surfactants and is likely to cause phase separation. Therefore application thereof to pharmaceuticals for external use, cosmetics, and agrochemical preparations is difficult.
In practical use, a spray preparation that can form mist capable of inclusion/sustained release of a plurality of compounds is required. For example, pharmaceuticals for external use and cosmetics may require dissolving both a hydrophilic compound and a hydrophobic compound, and chemical treatment is then needed to, for example, introduce a hydrophilic functional group into the hydrophobic compound. None of the conventional techniques mentioned above has suggested a material that fulfills both performance as a spray base material and practically requisite performance such as inclusion/sustained release of compounds.
On the other hand, hydrogel is used in various fields as a highly biocompatible material having water as a medium, and various studies have been conducted on hydrogel that includes a macromolecular compound and hydrogel that is obtained via self-assembly of low-molecular compounds.
Polymer gel (chemical gel and physical gel) that is obtained by crosslinking of macromolecular compounds via a covalent bond, an ionic bond, a hydrogen bond, or the like can prevent leakage of liquid from a container during spraying via solidification of solution which is useful. However, chemical polymer gel resulting from chemical crosslinking is difficult to spray as gel, and most of physical polymer gel is not sprayable. Even a sprayable polymer gel cannot achieve inclusion/sustained release of low-molecular compounds such as drugs, and its poor compatibility with the drugs, etc. and other additives leads phase separation, which is a problem.
In research on a low-molecular hydrogelator including a low-molecular compound, determination of a mechanism of self-organization among low-molecular compounds in water and molecular design are difficult, but the functions of the low-molecular hydrogelator have attracted a lot of interest, which lead active research on that in recent years. As a result, some low-molecular hydrogelators have been discovered [Non-patent Documents 1, 2]. Most of them are amphiphilic compounds that have both a long-chain alkyl group as a hydrophobic moiety and a hydrophilic moiety, and examples of these include ones having an amino acid [Non-patent Document 3], ones having a peptide [Patent Documents 7, 8], ones having a mono- or polysaccharide [Non-patent Documents 4, 5], and ones having a polyol [Non-patent Document 6], as the hydrophilic moiety. A low-molecular gelator has been developed utilizing the fact that a peptide including valine easily assumes a β-sheet structure [Non-patent Document 7].
Low-molecular hydrogelators that gel aqueous alcohol solutions and/or organic solvent aqueous solutions or that cannot gel water alone nor organic solvent alone but gel aqueous alcohol solutions and/or organic solvent aqueous solutions have been reported. A well-known common characteristic of low-molecular gel is to quickly react to external stress to convert from gel to sol.